Frequency switching circuit for multiple scan rate video display apparatus

Abstract

A video display apparatus operates at two or more horizontal scan rates. In order to prevent excessive collector voltage from being applied to the horizontal output transistor, a transition circuit turns off the horizontal output transistor when switching between horizontal scan frequencies occurs. The horizontal output transistor is maintained nonconductive until the correct oscillator frequency and supply voltage is coupled to the horizontal output circuit.

What is claimed is: 
     
       1. A multiple scan rate deflection circuit for a video display apparatus comprising: a line deflection oscillator selectably operable at first and second rates;   a source of first voltage;   a source of second voltage;   a line deflection winding;   a line deflection output stage incorporating output switching means coupled to said line deflection winding;   switch means having first and second positions, said first position causing said oscillator to operate at said first rate and causing said source of first voltage to be coupled to said output transistor, said second position causing said oscillator to operate at said second rate and causing said source of second voltage to be coupled to said line deflection output stage;   first means coupled to said oscillator for controlling the conduction of said output switching means to generate scanning current in said line deflection winding; and   second means coupled to said switch means and said conduction controlling means for causing said output switching means to be substantially nonconductive for a predetermined period of time when said switch means changes states between said first and second positions.   
     
     
       2. The arrangement defined in claim 1, wherein said means for causing said output switching means to be nonconductive comprises a resistor and a capacitor, said capacitor becoming discharged when said switch means changes states, said capacitor becoming charged when said switch means is in said first or second positions, said output switching means being nonconductive when the voltage across said capacitor is below a predetermined level. 
     
     
       3. The arrangement defined in claim 1, wherein said switch means comprises a mechanical switch having an intermediate open circuit position which activates said second means when said switch means is in said intermediate position and keeps said second means deactivated when said switch means is in said first and second positions. 
     
     
       4. A multiple scan rate deflection circuit for a video display apparatus comprising: a line deflection oscillator selectably operable at first and second line rates;   a line deflection output stage, coupled to said line deflection oscillator, including an output switch periodically switched between conductive and nonconductive states;   means coupled to said line deflection oscillator for causing said oscillator to change operation from one of said first and second line rates to the other line rate; and   means for rendering said output switch nonconductive for a predetermined time during the transition from said one to said other line rate, such that said switch cannot be rendered conductive during said predetermined interval.